Overview of the Ene Reactions

The process of transfer of one hydrogen atom from an allylic alkene (an ene) to an electrophilic alkene (an enophile) followed by formation of a new cr-bond and migration of allylic double bond is known as ene reaction or Alder ene reaction [1]. The ene may be an alkene, alkyne, allene, cyclopropane ring or arene, whereas an enophile may be an alkene, carbonyl, thiocarbonyl, imino or diazo compound having an electron withdrawing substituent. Oxygen may also serve as an enophile. The reaction usually takes place from left to right, since a new t bond is formed at the expense of the ir bond of the enophile as depicted below. 

These reactions resemble Diels-Alder reactions, with one of the n bonds of the diene is replaced by a ci bond in the allylic alkene. These reactions are also considered as intermolecular 1,5-hydrogen shifts. The Lewis acid catalyzed ene 

In entry 3, ally lie function in the open chain takes part in ene reaction because reaction with endocyclic double bond will produce less stable TS. 

When an unsaturated ketone in enol form undergoes ene reaction, the reaction is known as the Conia ene reaction [3]. The following examples [3-5] are illustrative. 

---